Deep sea diapirs and bottom simulating reflector in Fairway Basin (SW Pacific)

A recent swath-bathymetry and geophysical survey of the R/V L'Atalante in the Fairway Basin between Australia and New Caledonia allowed to confirm the Cretaceous age of the creation of the basin by continental stretching. This first stage of opening of the Fairway Basin is associated with the deposition of a continuous salt/mud layer feeding today numerous diapirs, some of them piercing the 3 to 4 km thick sedimentary cover and reaching the seafloor. In close link with this salt/mud layer a Bottom Simulating Reflector indicator of gas hydrates level occupies a 70000 km² surface at about 500 to 600 m-depth beneath the sea floor. The coexistence of both BSR and diapirs suggests a thermogenic better than biogenic origin for the gas hydrates horizon.     

Active oceanic spreading in the northern North Fiji Basin: Results of the NOFI cruise of R/V L'Atalante (newstarmer project)

The South Pandora and the Tripartite Ridges are active spreading centers located in the northern part of the North Fiji Basin. These spreading centers were surveyed over a distance of 750 km during the NOFI cruise of R/V L'Atalante (August–September 1994) which was conducted in the frame of the french-japanese Newstarmer cooperation project. SIMRAD EM12-dual full coverage swath bathymetric and imagery data as well as airgun 6-channel seismic, magnetics and gravity profiles were recorded along and offaxis from 170°40 E to 178° E. Dredging and piston coring were also performed along and off-axis. The axial domain of the South Pandora Ridge is divided into 5 first-order segments characterized by contrasted morphologies. The average width of the active domain is 20 km and corresponds either to bathymetric highs or to deep elongated grabens. The bathymetric highs are volcanic constructions, locally faulted and rifted, which can obstruct totally the axial valley. The grabens show the typical morphology of slow spreading axes, with two steep walls flanking a deep axial valley. Elongated lateral ridges may be present on both sides of the grabens. Numerous volcanoes, up to several kilometers in diameter, occur on both flanks of the South Pandora Ridge. The Tripartite Ridge consists of three main segments showing a sigmoid shape. Major changes in the direction of the active zones are observed at the segment discontinuities. These discontinuities show various geometrical patterns which suggest complex transform relay zones. Preliminary analysis of seismic reflection profiles suggest that the Tripartite Ridge is a very young feature which propagates into an older oceanic domain characterized by a significant sedimentary cover. By contrast, a very thin to absent sedimentary cover is observed about 100 km on both flanks of the South Pandora Ridge active axis. The magnetic anomaly profiles give evidence of long and continuous lineations, parallel to the South Pandora Ridge spreading axis. According to our preliminary interpretation, the spreading rate would have been very low (8 km/m.y. half rate) during the last 7 Ma. The South Pandora and Tripartite Ridges exhibit characteristics typical of active oceanic ridges: (1) a segmented pattern, with segments ranging from 80 to 100 km in length; (2) an axial tectonic and volcanic zone, 10 to 20 km wide; (3) well-organized magnetic lineations, parallel to the active axis; (4) clear signature on the free-air gravity anomaly map. However, no typical transform fault is observed; instead, complex relay zones are separating first-order segments.     

Modeling tropospheric wet delays with dense and sparse network configurations for PPP-RTK

GPS Solutions - Precise Point Positioning (PPP) is a well-known technique of positioning by Global Navigation Satellite Systems (GNSS) that provides accurate solutions. With the availability of...     

Variability of the axial morphology and of the gravity structure along the Central Spreading Ridge (North Fiji Basin): evidence for contrasting thermal regimes

The Central Spreading Ridge (CSR) is located in the central part of the North Fiji Basin, a complex back-arc basin created 12 Ma ago between the Pacific and Indo-Australian plates. The 3.5 Ma old CSR is the best developed, for both structure and magmatism, of all the spreading centers identified in the basin, and may be one of the largest spreading systems of the west Pacific back-arc basins. It is more than 800 km long and 50–60 km wide, and has been intensively explored during the French-Japanese STARMER project (1987–1991).The CSR is segmented into three first order segments named, from north to south, N160°, N15° and N-S according to their orientation. This segmentation pattern is similar to that found at mid-ocean ridges. The calculated spreading rate is intermediate and ranges from 83 mm/yr at 20°30 S to 50 mm/yr at 17°S. In addition, there is a change in the axial ridge morphology and gravity structure between the northern and southern sections of the CSR. The axial morphology changes from a deep rift valley (N160° segment), to a dome split by an axial graben (N15° segment) and to a rectangular flat top high (N-S segment). The Mantle Bouguer Anomalies obtained on the northern part of the CSR (N160°/N15° segments) show bull's eye structures associated with mantle upwelling at the 16°50S triple junction and also in the middle of the segments. The Mantle Bouguer Anomalies of the southern part of the ridge (N-S segment) are more homogeneous and consistent with the observed smooth topography associated with axial isostatic compensation.At these intermediate spreading rates the contrast in bathymetry and gravity structure between the segments may reflect differences in heat supply. We suggest that the N160° and N15° segments are cold with respect to the hot N-S segment. We use a non-steady-state thermal model to test this hypothesis. In this model, the accretion is simulated as a nearly steady-state seafloor spreading upon which are superimposed periodic thermal inputs. With the measured spreading rate of 50 mm/yr, a cooling cycle of 200,000 yr develops a thermal state that permits to explain the axial morphology and gravity structure observed on the N160° segment. A spreading rate of 83 mm/yr and a cooling cycle of 120,000 yr would generate the optimal thermal structure to explain the characteristics of the N-S segment. The boundaries between the hot N-S segment and its cold bounding segments are the 18°10 S and 20°30 S propagating rifts. A heat propagation event along the N-S segment at the expense of the adjacent colder failing segments, can explain the sharp changes in the observed morphology and structure between the segments.     

Theory of interaction intensities,buffer capacities,and pH stability in aqueous systems,with application to the pH of seawater and a heterogeneous model ocean system

The choice of convenient basic constituents for evaluating pH stability of aqueous systems is discussed, and two useful interaction parameters are defined and related to the buffer capacity: the interaction capacity, $\text{δ′}{}_{\mathrm{X,Y}}\text{=}\text{?}\text{p}\text{X}\text{?}\text{TOT}\text{Y}$, and the interaction intensity, $\text{δ}{}_{\mathrm{X,\; Y}}\text{=}\text{?}\text{p}\text{X}\text{?}\text{pTOT}\text{Y}$; for pH and TOTH, δ′_{H, H} = ?β_H^?1, where β_H is the pH buffer capacity. A method is presented for the computation of exact values of all interaction capacities and intensities through inversion of the Jacobian matrix of the system of non-linear equations describing the aqueous system. The major species of an aqueous system (H₂O, H⁺, solid phases, gases, and the most abundant solute species) are shown to constitute a useful set of basic constituents for evaluation of approximate pH buffer capacities according to a simple rule: the major-minor species rule for zeroth order pH-TOTH interaction. The concepts of buffering and pH-statting are examined and contrasted; it is demonstrated that the buffer capacity of an aqueous system cannot be infinite: it is limited by the concentration of solutes in solution. The effect upon pH of variations in constituents other than H⁺ is described in terms of first order interactions via complex formation and solid formation; approximate formulas for calculation are derived. Higher order interactions are derived from combinations of first order ones. The pH stability of the ocean system is examined in terms of an aqueous phase model including ion-association reactions and a heterogeneous model incorporating CO₂ in the gas phase, quartz, kaolinite, calcite, chlorite, and illite, in addition to the aqueous phase. There is an approximately three-fold enhancement of buffer capacity in the aqueous model as a consequence of ion-association. Only a few interaction pathways are of quantitative significance in establishing the buffer capacity. Results for the heterogeneous ocean model lend quantitative support to Sillén's notion of pH stability: the buffer capacity is about four hundred times greater than that of the aqueous phase model.     

Investigations of iron coordination and redox reactions in seawater using Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using ⁵⁹Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10⁶M⁻¹s⁻¹ and 20 M⁻¹s⁻¹, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH)₃^o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater.In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.     

Speleothem record of the last 180 ka in Villars cave (SW France): Investigation of a large δ18O shift between MIS6 and MIS5

The Vil-car-1 flowstone core from Villars cave (SW France) provides one of the first European speleothem records extending back to 180 ka, based on U–Th TIMS and MC-ICP-MS measurements. The core offers a continuous record of Termination II and the Last Interglacial. The penultimate deglaciation is characterized by a prominent 5‰ depletion in calcite δ¹⁸O. Determining which specific environmental factors controlled such a large oxygen isotopic shift offers the opportunity to assess the impact of various factors influencing δ¹⁸O variations in speleothem calcite.Oxygen isotope analyses of fluid inclusions indicate that drip water δ¹⁸O remained within a very narrow range of ±1‰ from Late MIS6 to the MIS5 δ¹⁸O optimum. The possibility of such a stable behaviour is supported by simple calculations of various effects influencing seepage water δ¹⁸O.Although this could suggest that the isotopic shift in calcite is mainly driven by temperature increase, attempts to quantify the temperature shift from Late MIS6 to the MIS5 δ¹⁸O optimum by assuming an equilibrium relationship between calcite and fluid inclusion δ¹⁸O yield unreasonably high estimates of ～20 °C warming and Late MIS6 cave temperatures below 0 °C; this suggests that the flowstone calcite precipitated out of thermodynamic equilibrium at this site.Using a method proposed by Guo et al. (submitted for publication) combining clumped isotope measurements, fluid inclusion and modern calcite δ¹⁸O analyses, it is possible to quantitatively correct for isotopic disequilibrium and estimate absolute paleotemperatures. Although the precision of these absolute temperature reconstructions is limited by analytical uncertainties, the temperature rise between Late MIS6 and the MIS5 optimum can be robustly constrained between 13.2 ± 2.6 and 14.6 ± 2.6 °C (1σ), consistent with existing estimates from Western Europe pollen and sea-surface temperature records.     

MHYDAS‐Erosion: a distributed single‐storm water erosion model for agricultural catchments

In this paper, we present MHYDAS‐Erosion, a dynamic and distributed single‐storm water erosion model developed as a module of the existing hydrological MHYDAS model. As with many catchment erosion models, MHYDAS‐Erosion is able to simulate sediment transport, erosion and deposition by rill and interrill processes. Its originality stems from its capacity to integrate the impact of land management practices (LMP) as key elements controlling the sedimentological connectivity in agricultural catchments. To this end, the water‐sediment pathways are first determined by a specific process‐oriented procedure defined and controlled by the user, which makes the integration of LMP easier. The LMP dynamic behaviours are then integrated into the model as a time‐dependent function of hydrological variables and LMP characteristics. The first version of the model was implemented for vegetative filters and tested using water and sediment discharge measurements at three nested scales of a densely instrumented catchment (Roujan, OMERE Observatory, southern France). The results of discharge and soil loss for simulated rainfall events have been found to acceptably compare with available data. The average R² values for water and sediment discharge are 0·82 and 0·83, respectively. The sensitivity of the model to changes in the proportion of LMP was assessed for a single rain event by considering three scenarios of the Roujan catchment management with vegetative filters: 0% (Scenario 1), 18% (Scenario 2, real case) and 100% (Scenario 3). Compared to Scenario 2 (real case), soil losses decreased for Scenario 3 by 65% on the agricultural plot scale, 62% on the sub‐catchment scale and 45% at the outlet of the catchment and increased for Scenario 1 by 0% on the plot scale, 26% on the sub‐catchment scale and 18% at the outlet of the catchment. Copyright © 2010 John Wiley & Sons, Ltd.     

Evaluating the persistence of post‐fire residual patches in the eastern Canadian boreal mixedwood forest

In boreal forest ecosystems, wildfire severity (i.e. the extent of fire‐related tree mortality) is affected by environmental conditions and fire intensity. A burned area usually includes tree patches that partially or entirely escaped fire, called ‘residual patches’. Although the occurrence of residual patches has been extensively documented, their persistence through time, and thus their capacity to escape several consecutive fires, has not yet been investigated. Macroscopic charcoal particles embedded in organic soils were used to reconstruct the fire history of 13 residual patches of the eastern Canadian boreal mixedwood forest. Our results display the existence of two types of residual patches: (i) patches that only escaped fire by chance, maybe because of local site or meteorological conditions unsuitable for fire spread (random patches), and (ii) patches with lower fire susceptibility, also called ‘fire refuges’ that escaped at least two consecutive fires, probably because of particular site characteristics. Fire refuges can escape fire for more than 500 years, up to several thousand years, and probably burn only during exceptionally severe fire events. Special conservation efforts could target fire refuges owing to their old age, long ecological continuity and potential specific biological diversity associated to different microhabitats.